When paper is held to a platen by vacuum rather than clamps at the leading edge of the paper, there must be a way to determine when the leading edge of the paper passes the printing station. Otherwise, there is no way to synchronize the message to be printed on the paper with the dimensions of the paper. This problem arises because vacuum attraction of paper onto a platen is an asynchronous operation on which the leading edge of the paper can be picked up by the platen at any rotational position. Additionally, since vacuum can pick up the paper at any rotational position of the platen, it is desirable not to waste vacuum or otherwise permit excessive amounts of air to dissipate the vacuum in order to promote firm gripping of the paper onto the platen.
Therefore, it is an object of the present invention to minimize the passage of air into a vacuum-operated, record-carrying platen.
It is another object of the present invention to determine the location of the leading edge of the paper wrapped around a vacuum platen.
Still another object of the invention is to control the phasing of the printing operation on a sheet of paper wrapped around a rotating platen.
It is yet another object of the present invention properly to phase removal of vacuum from the drum for ejecting paper from the rotating platen.